I'm looking to create a setup which has storage additional to the SD Card. I'm thinking of using a 'self-powered' (i.e. off the usb power, not externally powered) USB drive (e.g. the "passport" drives). For game emulation machine. Drive will store roms and associated media. A few questions:

That depends on the current requirements of the drive. I have found most USB HDDs that I have tried with a Pi2 or 3 have worked fine when using the official PSU.

b) Any difference between using a USB2 and USB3 drive. The USB3 drive appear to be better value for money (1TB drives coming in around £60)

Again it depends on the requirements of the drive. I am happily using USB3 2TB drives with a Pi2 and a PI3. (Which were £62 in Argos, but are now more expensive with some useless bundled "free" software)
I've no idea on the last two questions, sorry.